Metallic effect pigments have been used for many years in coatings in order to generate a metallic effect.
Conventional metallic effect pigments consist of platelet-shaped metallic pigments whose effect derives from the directed reflection of incident light at the metallic pigments of planar form which are oriented in parallel in the respective application medium.
Typical fields of application of the metallic effect pigments are the coatings industry, especially the automotive industry, the printing industry, and the plastics industry.
The metallic effect is described by certain parameters. These parameters include the brilliance, characterized by sparkle and metallic luster, the lightness, and the flop, i.e., the change in lightness as a function of incident angle and/or viewing angle, and the covering power. In the case of colored metallic coatings, further parameters are the chroma and the color flop (“two-tone”). The luster is determined by the ratio of reflected light to scattered light in relation to a standard.
Key factors influencing the metallic effect include the particle morphology and the form factor, i.e., the ratio of average particle diameter to average particle thickness, of the pigments, the thickness of the particles, and also their surface roughness, the particle size, the particle-size distribution, and the orientation of the pigment parallel to the surface of a coating material or plastic.
In metallic effect pigment particles with relatively large diameters and uniform morphology, reflection is relatively high, and this is manifested in high metallic brilliance, improved lightness, and strong flop, whereas, for pigments with relatively low particle diameters, the scattering fraction is very high, leading to good covering power.
The covering power is determined above all, however, by the thickness of the metallic pigments. The thinner the metallic pigments, the better their specific covering power, i.e., the covering power per unit weight.
On the part of the printing, coatings, plastics, and cosmetics industries there is a great interest in metallically dark metallic effect pigments.
For relatively high-value applications, particularly thin aluminum effect pigments have been developed, and are produced via PVD techniques.
Metallic effect pigments produced by PVD techniques have been known for some considerable time. They are notable for extremely high luster, immense covering power, and unique optical properties. Owing to their low thickness of around 30 to 70 nm and their extremely smooth surfaces, they have a tendency, following application, to conform very closely to the substrate. If the substrate is very smooth, the result is virtually a mirrorlike appearance.
Of the pure metallic effect pigments, only aluminum effect pigments have been disclosed commercially to date. Examples thereof are Metalure® (manufactured by Avery Dennison, sold by Eckart), Decomet® (Schlenk) or Metasheen® (Ciba).
Such pigments represent the “silver” hue in its highest embodiment.
Pigments produced by PVD techniques on the basis of metallic layers are described in more detail in U.S. Pat. No. 2,839,378. Described therein is the manufacture of mirrorlike pigments with extremely thin layer thicknesses, which are applied by vapor deposition to a substrate provided with a “release layer”. After the metal layers have been applied and the film has been detached, the pigments are comminuted to the desired particle size by means of mechanical action.
The application of pigments manufactured in this way in coating formulations is described in detail in U.S. Pat. No. 2,941,894. That patent emphasizes the high reflectivities, the low level of pigmentation, i.e., a low level of pigment concentration in the application medium, and the high specific hiding power or covering power of the pigments.
A method for producing metallic pigments by means of vapor-deposition techniques with a thickness of 35 to 45 nm is described in greater precision in U.S. Pat. No. 4,321,087 and entails the application of a release layer, the metalizing operation, the detachment procedure in a solvent bath, the concentrating of the detached particles, and the comminution of the particles to the desired pigment size by means of ultrasound.
WO99/35194 describes pigments having a three-layer construction, in which the inherent color of an intermediately situated metal layer is not altered by two external dielectric support layers. A disadvantage with this production method is that it involves three layers to be applied by vapor deposition in order to be able to produce the multilayer construction, with the production costs being greatly increased as a result.
EP 1 522 606 A1 describes the production of a sheet with black aluminum oxide. In this case, neither effect pigments nor multilayer structures are disclosed. The sheets disclosed here have no notable metallic effect with luster and flop.
U.S. Pat. No. 4,430,366 describes the production of sheets comprising a mixture of metal and metal oxide. Here, again, no effect pigments are referred to.
WO 2007093401 A2 describes dark metallic effect pigments comprising a layer with a largely homogeneous composition in terms of oxygen and metal. A disadvantage is that the method for producing these effect pigments is costly and inconvenient.
DE 69601432 T2 relates to a method for thermal generation of an image on a substrate, in which an oxygen-containing, black aluminum layer is applied in such a way that it has an optical transmittance of at least 0.3 at a wavelength between 200 and 1100 nm. This document does not relate to the provision of effect pigments.
EP 1 144 711 B1 discloses a method for producing reflective color pigments, in which, atop a reflection layer, at least one layer is applied which produces a color change, said layer comprising a transparent material having a refractive index of greater than 1.8, typically metal oxide, and a light-absorbing metal, application taking place with simultaneous evaporation, and the light-absorbing metal being different from the metal of the metal oxide. In terms of process engineering, the method is very difficult to control.
DE 10 2007 007 908 A1 discloses dark metallic effect pigments produced by PVD techniques. They have a largely homogeneous composition and possess a relatively high oxygen content of 25 to 58 atom %. The layer is dark, since the metal is in the form of small metal clusters dispersed in metal oxide. Effect pigments of this kind produce dark but highly lustrous effect pigments with a pronounced light/dark flop. The method for producing these PVD metallic effect pigments with a homogeneous composition is likewise costly and inconvenient in terms of process engineering, and permits only low production rates.
In order to ensure the high oxygen content and the homogeneous chemical composition, it is necessary to exert extensive monitoring, during the coating operation, of the metal vaporization rate and the oxygen supply rate over both the width and the length of the coating belt. This necessitates a very high level of cost and complexity particularly in a production plant: for instance, over the belt width, for example, the metal layer thickness can be measured in situ by means of transmittance measurements, and adjusted. It is very difficult, however, to monitor the operation over the entire vapor-deposition zone, i.e., over width and length of the belt.